In quantitative positron emission tomography (PET) imaging, the attenuation background (i.e., map) of the subject (e.g., a human patient or an animal) must be obtained or estimated to accurately reconstruct the radioactive tracer distribution. A PET image reconstructed with an incorrect attenuation map or with no attenuation map would suffer from significant artifacts. Existing methods for estimating the attenuation map of a PET image can be categorized into two general classes: transmission and non-transmission.
Transmission methods are based on transmission data acquired with an external radionuclide source, computed tomography (CT), or magnetic resonance imaging (MRI) scan. In a PET/CT system, attenuation correction is achieved with the CT sub-system. However, there are significant mismatches between a CT image at an average energy of 60-80 kilo-electron-Volts (keV) and a PET attenuation background at 511 keV, and their physical features are quite different, which can cause substantial artifacts. When a correlation map between CT and PET linear attenuation coefficients is established, low dose CT can be used for PET attenuation correction. To compensate for respiratory motion, a long CT acquisition time can be used to obtain an averaged CT attenuation, and a respiratory-gated CT acquisition can help reduce motion artifacts.
Non-transmission methods derive an attenuation map without a separate transmission scan. In the case of brain or abdominal imaging, in which soft tissue is the dominant constituent, a uniform attenuation coefficient is assigned within the tissue contour. In some non-transmission methods, the consistency condition of the Radon transform can be used to estimate regions of fairly uniform attenuation. In some methods, a discrete consistency condition can be utilized for the same purpose. Also, some methods attempt to reconstruct the activity and the attenuation simultaneously based on a statistical model. However, all of these techniques suffer from cross-talk artifacts.